Write a changelog

Release notes for early releases have been taken from the GitHub
releases. For the unreleased things on master I read the commit messages
and picked the most significant ones.

CHANGELOG.md

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+# Changelog
+
+## Unreleased
+
+Fixes:
+
+ * Fixed various warnings on MSVC. (#60)
+ * Fixed an occurrence of arithmetic overflow.
+ * Fixed a bug in command-line argument parsing.
+
+Improvements:
+
+ * Memory usage has been reduced. (#148, #156, #213)
+ * A fuzzing target has been added.
+ * `--` can now be used to stop parsing command-line arguments.
+ * Builds are now tested on Travis CI and Appveyor. (#136, #191)
+ * Improved error messages.
+ * Improved build instructions.
+
+## v1.0.1
+
+Released 2017-03-21.
+
+Fixes:
+
+ * Fixed yet another crash for small images. (#38)
+ * Makes metadata handling consistent. (#100)
+ * Fixed some incorrect error messages. (#40)
+ * Fixed various build issues (a.o. removes the gflags dependency).
+ * Add missing documentation.
+
+Improvements:
+
+ * Adds memory limit support. (#75)
+
+## v1.0
+
+Released 2017-03-15.
+
+Fixes:
+
+ * Fixed two crashes. (#29 and 5de9ad8)
+ * Made memory requirements more explicit.
+
+## v0.2
+
+Released 2016-01-17.
+
+Fixes:
+
+ * The same filename can now be used for input and output. (#20)
+ * Various compilation issues were fixed. (#12, #14)
+ * Fixed a crash on Windows due to usage of uninitialized memory. (#23)
+
+Improvements:
+
+ * 64-bit binaries for Windows are now released.
+
+## v0.1
+
+Released 2016-12-21.
+
+Fixes:
+
+ * Fixed generated jpegs which some viewers were unable to open. (#1)
+ * Fixed the build system for Windows.
+
+## v0
+
+Released 2016-10-21.
+
+Initial release.